Waveguide harmonic suppressor employing subsidiary waveguides, cut off for fundamental, for coupling main waveguide harmonics to absorber



June 1, 1965 wANTucH ,187,277

E. 3 WAVEGUIDE HARMONIC SUPPRESSOR EMPLOYING SUBSIDIARY WAVEGUIDES. CUT OFF FOR FUNDAMENTAL, FOR COUPLING MAIN WAVEGUIDE HARMONICS TO ABSORBER Filed May 14, 1962 aw' I i gi @pw new United States Patent @ffice 3,187,217 Patented June 1, i955 WAVEGUIDE HARMGNIC SUPPRESSOR EMPLOY- ING SUBSIDIARY WAVEGUIDES, CUT @FF FR FUNDAMENTAL, FOR COUPLING MAIN WAVE- GUKDE HARMNCS T0 ABSORBER Ernest Wantuch, Livingston, NJ., assigner to Airtron, Inc., Morris Plains, NJ. Filed May 14, 1962, Ser. No. 194,282 11 Claims. (Cl. S33-43) This invention relates to microwave harmonic suppressors.

A waveguide normally operates at a fundamental frequency and mode; however, higher harmonics may be applied to the waveguide in various modes by transmitters such as magnetrons or high power klystrons. For many known reasons, it is often desirable to eliminate these higher harmonics. The usual arrangements for suppressing harmonic include a series of apertures along one side wall of the waveguide and dissipation material located near the apertures for dissipating microwave energy coupled through the apertures.

The prior art harmonics Suppressors frequently do not provide sufficient attenuation for the harmonics and introduce unnecessary loss into the main waveguide channel. In some cases, the Suppressors have exhibited undesirably high reflection for the higher harmonics, and in other cases, some of the many modes in which the harmonic energy is present may not be attenuated to any significant extent.

Accordingly, a principal object of the present invention is to increase the attenuation of harmonics and reduce the attenuation of the dominant or fundamental waveguide mode, in waveguide harmonic Suppressors. Another object of the invention is to minimize the size of harmonic Suppressors having these properties. Additional objects include reduction of reflections in the harmonic or stop band of frequencies, and increasing the attenuation for all the modes in which harmonic energy may propagate.

In accordance with one illustrative embodiment of the invention, a main waveguide channel is provided with a series of apertures on all four sides, and each aperture is provided with an associated short waveguiding passageway. Subordinate microwave chambers containing lossy material may extend parallel to the main waveguide, and many of the small waveguiding passageways may be coupled to a single one of these subordinate lossy chambers. Dielectric rods may extend through the passageways to more strongly couple harmonics from the main waveguide to the subordinate waveguide chambers where the harmonic energy is dissipated. By extending the rods physically into the main waveguide, or the lossy subordinate chambers, or both, much greater attenuation of the harmonic energy is achieved than has been possible heretofore.

The dielectric rods are preferably made of a material having a very low loss such as quartz so that the material extending into the main waveguide channel does not significantly increase the attenuation of the fundamental mode. The use of dielectric material in the passageways has the collateral advantage of permitting the diversion of harmonic waves through waveguide passageways of relatively small cross-section. Accordingly, a great many of the branch passageways may be packed along a relatively short length of the main waveguide. High attenu- Awhich microwave energy lmay propagate.

ation of harmonics with a relatively compact Structure iS therefore achieved.

In the case of one representative harmonic suppressor, fifteen inches in length and operating at C-band frequencies in the range of 4.4 to 5.0 kilomegacycles, the second harmonic of the fundamental was reduced by 40 decibels, or 99.99%. The attenuation for the fundamental frequency in the case of this representative harmonic suppressor was less than one-quarter of one decibel.

In accordance with an important feature of the invention, a rectangular waveguide is provided with four spaced subordinate lossy waveguiding chambers, and a set of dielectric-filled conductively bounded passageways interconnects each of the four walls of the main waveguide with the subordinate channels. n

In accordance with another feature of the invention, a harmonic suppressor is provided with a main waveguiding passageway and a subordinate lossy chamber, and dielectric elements extending into the main waveguide to couple microwave energy to the lossy chamber.

Another featured construction involves the use of branch passageways on all four sides of a main waveguide to provide harmonic suppression which is virtually independent of the modes in which the harmonics are propaeating- The novel features which are believed to be characteristic of the invention, both as to its organization and mode of operation, together with further objects, features and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which illustrative embodiments of the invention are disclosed, by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only and does not constitute a limitation of the invention.

In the drawing:

FIG. l is an isometric view of a harmonic suppressor illustrating the principles of the present invention; and

FIG. 2 is a cross-sectional view of a modified version of the invention.

Referring more particularly to the drawings, FIG. 1 shows a harmonic suppressor having a principal waveguide channel 12. The principal channel 12 is provided with coupling flanges 14 and 16 at its two ends to facilitate connection to the other components ofV a microwave system.

On each of the four sides of the waveguide channel 12, harmonic suppression structures 18, 2l), 22 and 24 are provided. As these four structures are generally similar, only the structure 22 associated with the upper broad wall of waveguide 12 will be considered in detail. The structure 22 includes an outer conductive Wall 26, shown cut away in FIG. 1 to reveal the operative inner components. The operative harmonic suppression components within the conductive wall 26 includes a strip of corrugated lossy material 28 mounted against the upper surface of the harmonic suppressor structure 22. Immediately below the lossy dielectric strip 2S is an open space through This space, together with material 28, provides a subordinate lossy microwave channel or chamber which extends generally 12 to the lossy chamber near the top of the structure 22.

' In Fig. l, these microwave passageways are implemented by openings in the upper wall 30 of the waveguide I2 and by a series of hollow conductive tubes 32.` As discussed below in connection with FIG. 2, the conductive passageways may be implemented by using a single thicker member of conductive material with holes drilled through it.

Dielectric rods 34 extend through the conductive passageways from the waveguide 12 to the space below the lossy strip 2S. It has been found that coupling to the harmonics which are present in the main guide 12 is greatly enhanced by extension of the dielectric rods 34 into the main waveguide channel. Thus, for example decibel more coupling of the harmonic energy is obtained waveguide, as contrasted with an arrangement in which the dielectric rods were flush with the inner wall of thek waveguide.

In order to provide close packing of the openings coupling the main waveguide to the harmonic dissipation space above the quartz rods, the openings may be staggered, with two holes present in the first row and three holes being provided in the second row. This arrangement of alternate and staggered holes is continued along the length of the upper surface of the waveguide. A similar arrangement of closely-packed passageways is provided on the lower surface of waveguide 12. On each of the two narrower sidewalls of the waveguide 12, spacing requirements limi-t the number of harmonic suppression passageways. Accordingly, the tubes 36 are arranged at aV considerable angle and successive pairs of tubes are mounted to form closely-packed arrays along each of the narrow sidewalls of waveguide 12. The lossy material 58 and 40 associated with the harmonic suppression structures and 24 may also be seen in FIG. 1.

FIG. 2 is a cross-sectional view through a slightly modified version of the device shown in FIG. 1.y Primed reference numerals in FIG. Z correspond to `unprimed numerals in FIG. l. central waveguide channel 12 and the four associated waveguide suppression structures 18', 20', 22 and 24. The structure FIG. 2 also includes lossy material 28' associated with suppression structure 22, and additional lossy material 38 and 4G associated with waveguide suppression structures 20 and 24. The quartz rods 24' of FIG. 2 serve to couple harmonic microwave energy from the main guiding passageway 12 to thesubordinate lossy microwave chamber above rods 34'. One difference between the embodiments of FIGS. 1 and 2 involvesV the use of a solid strip of material such as strip 44 having a series of holes drilled through -it to accommodate the quartz rods 34, on each side of the main waveguide.

YThis is in contrast to the arrangement of FIG. 1 in which a number of conducting tubes 32 are employed to provide the conductive passageways. The secondV principal difference `involves completely filling the space around the outer ends of the dielectric rods with thermosetting lossy material, which is poured into the four outer chambers, after the dielectric rods are in place. In FIG. 2, the section is taken tok include quartz rod 46, and an adjacent quartz rod 48 is shown slightly behind the quartz rod 46 to indicate Vthe offsetting which is present in the coupling apertures in the narrow side walls of the waveguide 12'.

To fully understand the mode of operation of the present harmonic suppressor, it is useful to consider a specific example. In the present case, a C-band isolator will be considered. The main channel 12 for C-band has dimensions of 1.872 inches for :the broader side walls,V

and 0.872 inch for the narrow side walls. The recommended frequency range for C-band waveguides is 3.95 to 5.85 kilomegacycles, in accordance with Table II which appears at page 6-6 in the `Radio Engineering Handbook, Fifth Edition, 1959, by Keith Henney, Editorin-Chief, McGraw-Hill Company, Inc., New York. For the particular application, a frequency band of from l4.4 to 5.0 kilomegacycles was primarily of interest. Sys- The device of FIG. 2 includes the tem requirements necessitated suppression of second and higher harmonics present in the microwave signals. Accordingly, the branch waveguide channels must not transmit signals below 5.85 kilornegacycles to any substantial extent, but they must readily transmit harmonic signals which would have a frequency of at least twice the lowest frequency in the band. Thus, the branch passageways must readily couple energy of frequencies higher than twice the 3.95 kilomegacycles lower frequency which is recommended for C-band waveguides. Multiplying 3.95 2 gives a desired maximum cutoff frequency of 7.9 kilomegacycles. The branch waveguide passageways therefore must have a cutoff frequency below this 7.9 kilomegacycle frequency and above the maximum frequency of 5.85 kilomegacycles which is recommended for C-band waveguides.

The cutoff wave length for circular waveguides is given by the following formula:

where Lc is the cutoff wave length, D is the diameter of the waveguide passageway, and e is the dielectric `constant of the material in the waveguide.

In the present case, quartz rods having a diameter of one-half inch werey employed. Quartz has a dielectric constant of 4. Substituting in Equation l the cutoff wave length is determined to be 4.25 centimeters.Y To compute the cutoff frequency, the following formula may be used:

where L is equal to the Wave length, f is the frequency, and 3 1O10 cms/sec. is the speed of light and of microwave energy propagation in free space.

Using this formula, the cutoff frequency may be readily calculated to be about 7 kilomegacycles. Thus, the branch passageways will transmit signals of about 7 kilomegacycles and higher frequencies, but will not transmit lower microwave signals.

As mentioned generally above, a harmonic suppressor in which the suppression structures 1S, 20, 22 and 24 were fifteen inches long exhibited 40 decibels suppression for the second harmonic, corresponding to the elimination of 99.99 percent of this undesired higher frequency energy. Despite this high suppression of the second harmonic, this structure only introduced one-quarter of one decibel of loss for signals in the fundamental mode in the waveguide y12.

It is interesting-to note some of the advantages which are provided through the usc of quartz rods for coupling. Initially, the dielectric constant for quartz is four, as contrasted with the dielectric constant of one, for air. If the tubes 32 of FIG. 1 were not filled with quartz, the cutoff frequency of the one-half inch tubes would be raised from 7 kilomegacycles to 14 kilomegacycles. Under these circumstances, of course, no suppression of the second harmonic signals would occur. In order to provide harmonic suppression, much larger diameter airfilled passageways would have to be provided. Accordingly, through the use of quartz rods, the diameter may be reduced and the packing density of the passageways may be greatly increased. This permits high coupling to third and higher order harmonics, in addition to the second harmonic which may be present in waveguide 12. Quartz is also desirable in view of its very low loss. Thus, it can be extended into the waveguide 12 to extract second and higher harmonics, without attenuating the dominant mode to any significant extent.

With regard to materials employed in the harmonics suppressor ofthe present invention, a wide selection is available. Thus, the conductively bounded Waveguiding L f=3 1010 cms/sec.

surfaces may be `of copper, aluminum, or of any of a tures, they may be formed of any known microwave loss material. One typical material which may be employed is carbon-loaded epoxy. As mentioned above, quartz is preferred for the dielectric material in the branch passageways; however, other dielectric material such as alumina, and more common dielectric substances, may also be employed.

yIt is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. Thus, by way of example and not of limitation, the materials may be changed as noted above and the geometry of the device may be altered to provide the necessary cutoff frequency for proper harmonic suppression at any desired frequency range, through the use of the formulae and the textbook material referred to above. Rectangular or other yshapes of branch passageways may be employed instead of circular channels. With regard to another variant, when there is no objection to the radiation of microwave energy and its dissipation in the atmosphere, the small branch microwave passageways may be open-ended, and no lossy material is required. Accordingly, from the foregoing remarks, it is to be understood that the present invention is limited only by the spirit and scope of the appended claims.

In the claims:

1. A balanced microwave harmonic suppressor comprising:

a main rectangular waveguide,

means including microwave loss material for dissipating harmonic energy,

a plurality of closely-spaced apertures in each of the four walls of the main waveguide,

conductively bounded waveguiding passageways having a significantly smaller cross-section than said main waveguide coupling harmonic energy from said apertures to said dissipating means, and

dielectric material mounted in said passageways and protruding into said main waveguide.

2. A balanced microwave harmonic suppressor comprising:

a main rectangular waveguide,

means including microwave loss material for dissipat- Iing harmonic energy,

a plurality of apertures in each vof the four Walls of the main waveguide, and

subsidiary waveguides having a significantly ysmaller cross-section than said main waveguide coupling harmonic energy from said apertures to said d-issipating means, the length of said subsidiary waveguides being greater than their width, whereby they `are beyond cut-off for the principal mode in said main waveguide.

3. In a microwave harmonic suppressor, a main section of conductive waveguide, a lossy waveguide space adjacent said waveguide, a plurality of f conductively bounded passageways extending between said waveguide and said lossy space, the length of said passageways being greater than their largest transverse dimension, and dielectr-ic material located in said passageways.

4. In a microwave harmonic suppressor a main section of conductive waveguide, means including a plurality of conductively bounded passageways of reduced cross-section connected to said waveguide for eliminating harmonic energy from said waveguide, and means consisting solely of substantially lossless dielectric material located in said pa'ssageways and protruding into said main waveguide for coupling the harmonics from said main waveguide into said passageways without disturbing the principal mode in said m-ain waveguide.

5. A microwave harmonic suppressor comprising:

a ma-in rectangular waveguide,

means including microwave loss material 'for dissipatling harmonic energy, a plurality of apertures in each of the four walls of the main waveguide, and

means including subordinate waveguides having a sig niiicantly smaller cross-section than said main waveguide for coupling harmonic energy from said apertures topsaid dissipating means, the length of said Iwaveguides being greater than their width, whereby they are beyond cut-ott for the principal mode in said main waveguide.

6. 'In a microwave harmonic suppressor, a main sect-ion of conductive waveguide, a lossy waveguide space adjaject said waveguide, a plurality of conductively bounded passageways extending between said waveguide and said lossy space, and dielectric material located in said passageways and extending into said main waveguide.

7. A microwave harmonic suppressor comprising:

a main rectangular waveguide,

means including microwave loss material for dissi- -pating harmonic energy,

a plurality of apertures in each of the four walls of the main waveguide,

conductively bounded waveguiding passageways having a significantly smaller cro-ss-section than said main waveguide coupling harmonic energy from said yapertures to said dissipating means, the length of ysaid passageways being greater than their width, 'whereby they form waveguides which are beyond cut-oi for the principal mode in said main waveguide, and

dielectric material located in said passageways.

8. In a microwave harmonic suppressor, a main section of conductive waveguide, a lossy waveguide space adjacent said waveguide, a plurality of conductively bounded passageways extending between said waveguide tand said lossy space, said passageways having a length which is greater than their width, and quartz rods 1ocated in said passageways. y

9. In a microwave harmonic suppressor, a main section of rectangular conductive waveguide, means for dissipating microwave energy located adjacent said waveguide and a plurality of conductivity bounded subsidiary waveguides extending from all four walls of said main waveguide to said microwave dissipation means, the length of said waveguides being greater than their width, whereby they are beyond cut-olf for the principal mode in said main waveguide.

lil. In a microwave harmonic suppressor, a main section of conductive waveguide, a subordinate waveguide space adjacent said main waveguide, a plurality of conductively bounded passageways extending between said main waveguide and said subordinate waveguide space, dielectric rods extending through said p-assageways into said lsubordinate waveguide space, and lossy material substantially iilling said space and surrounding the ends of said dielectric rods.

11. A balanced microwave harmonic suppressor comprising:

a main rectangular waveguide;

lfour waveguiding channels extending along and parallel to said main waveguide on each of the four sides of said main waveguide;

lossy material located in each of said four waveguiding channels;

a plurality of rows of subsidiary conductively bounded -waveguiding passageways, extending from each side wall of said main waveguide to the associated channel containing lossy material, said passageways having a length greater than their maximum transverse dimensions; and

quartz rods extending through said passageways from said subordin-ate channels and protruding slightly into lsaid main rectangular waveguide.

(References on following page) References Cited by ythe Examiner UNITED STATES PATENTS King Ring Bittner Crawford Farr Tomiyasu 8 V2,909,735 10/59 Hessian..V 333-9 2,945,193 7/60 Strom S33-10 OTHER REFERENCES Regan: Microwave Transmission Circuits, Radiation Laboratory Series, McGraw-Hill, New York, copyright 1948, TK 6553R34.

HERMAN KARL SALLBACH, Primary Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO 5 18.7 7 '77 June l, 1965 Ernest Wantuch It s hereby certified that error appears in the above numbered patent reqlring correction and tha t the said Letters Patent should read as oorrectedbelow.

Column 6, line 43, for "conductivity" read conductively Signed and sealed this 12th day of October 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Allcsting Officer Commissioner of Patents 

3. INA MICROWAVE HARMONIC SUPPRESSOR, A MAIN SECTION OF CONDUCTIVE WAVEGUIDE, A LOSSY WAVEGUIDE SPACE ADJACENT SAID WAVEGUIDE, A PLURALITY OF CONDUCTIVELY BOUNDED PASSAGEWAYS EXTENDING BETWEEN SAID WAVEGUIDE AND SAID LOSSY SPACE, THE LENGTH OF SAID PASSAGEWAYS BEING 